This invention relates to an apparatus for measuring the density (thickness) of a sliver bundle at a forwardly tapering sliver guide of a drafting frame as the sliver bundle runs through the sliver guide. The sliver guide gathers a plurality of incoming slivers into a single bundle which passes through a roll pair downstream of the sliver guide as viewed in the direction of sliver advance. A movable, biased sensor element cooperates with a stationary counterface and defines therewith a constricted location for the throughgoing sliver. The change in position of the sensor element in response to variations in the sliver density affects a transducer for generating a control pulse.
In a known apparatus of the above-outlined type, an arrangement for guiding the slivers at the drafting frame input is provided. The arrangement includes a conically tapering sheet metal support tray for the slivers, provided with laterally upwardly bent wall surfaces, downstream of which there is situated the sliver guide having a rectangular inlet cross section as well as closely arranged top and bottom surfaces and conically converging lateral surfaces. The side-by-side arranged, inputted slivers glide off the tray surface formed of the sheet metal support tray as well as the bottom surface of the sliver guide. In the inlet zone between the inputted slivers and the lateral walls an intermediate space is present. The sliver guide is situated immediately upstream of a withdrawing roll pair whose parallel axes are oriented vertically. The roll pair simultaneously serves for measuring the sliver thickness within a predetermined tolerance range and the rolls may vary the distance from one another as a function of the thickness of the sliver undergoing measurement. The movable, spring-biased roll functions as a movable sensor element and is displaceable in a horizontal direction relative to the other, stationarily held roll. The stationary roll is formed of three discs arranged in an axial series. The middle disc has a lesser diameter than the two flanking discs whereby the roll periphery has a circumferential groove (grooved roll). The spring loaded movable roll is formed of a single disc which, along its circumferential zone extends into the groove of the stationary roll. The circumferential face of the middle disc of the grooved roll forms the stationary counterface for the circumferential face of the movable tongue roll. By means of the groove-and-tongue construction a constricted location of essentially rectangular cross-sectional configuration is formed through which the sliver composed of the gathered (compressed) slivers is passed for measurements.
In operation of the above-outlined arrangement, the individual slivers run into the sliver guide at the inlet of the drafting frame with a speed of, for example, 150 m/min. By virtue of the conically converging wall configuration of the sliver guide, the slivers are gathered into one plane in a side-by-side position without clamping. The slivers exiting the sliver guide are first densified by passing through the nip of the downstream-arranged roller pair, that is, they are compressed essentially to the material cross section and thus air is expelled from the slivers so that sliver measurement can take place. The circumferential speed of the rolls and the running speed of the sliver are identical, and thus no speed difference between the roll pair and the slivers exists. The clamping effect which is necessary for the sliver withdrawal from the sliver guide is utilized simultaneously for densification for measurement. After the sliver exits the nip of the roller pair, the slivers again separate from one another in the lateral direction and enter the after-connected drafting frame.
The above-outlined known apparatus has several disadvantages: it is structurally complex and expensive, and the groove-and-tongue configuration of the measuring rolls has to be manufactured with precision to ensure an exact fit. Further, the rolls have to be precisely aligned during assembly. The central disc of the grooved roll and the tongue roll also require high manufacturing accuracy (small tolerances): the diameter of the two discs must be identical to ensure that the circumferential speed of the two components is the same to avoid a drafting (stretching) of the slivers. Further, the two discs must have a high-precision circular run to avoid measuring errors. Even a small misalignment between the grooved roll and the tongue roll causes measuring errors.
It is a further disadvantage of the above-outlined known constructions that upon acceleration and deceleration of the rolls centrifugal forces are generated which lead to measuring errors. It is also a disadvantage that the drive for the two rolls is complex, particularly because the rotary drive needs to be designed for use with an axially shiftable roll and that both rolls of the roll pair have to be driven rolls. The drive for the displaceable (pivotal) roll includes a spur gear pair, one gear of which is mounted on the shaft of the roll whereas the other gear is arranged coaxially to the pivot axis of the pivot arm for the shiftable roll. As a result of this arrangement the gear pair remains in a meshing relationship even during pivotal motion of the arm. To obtain the required, oppositely oriented rotation of the two rolls, a further gear has to be used as an intermediate gear which, apart from a complex and expensive structure, disadvantageously introduces a relatively large play between teeth of meshing gears which may lead to significant inaccuracies.